The gene defective in Huntington's disease (HD), IT15, encodes a proteins, huntingtin, with unknown function. There is no apparent difference in IT15 RNA expression in control and HD brain, suggesting that the mutation acts at the protein level to cause HD. How the mutation, a polyglutamine expansion at the N-terminal of the protein, affects the properties of hungingtin expression, subcellular localization, and protein interactions within the cell is unknown. Curiously, the expression of IT15 occurs in all tissues, but the disease process affects striatal spiny neurons preferentially. These neurons possess unusual properties which make them vulnerable to the mutation. At present, almost nothing is known about huntingtin in relation to neuropathological changes in HD, the timetable in which the protein is expressed in developing striatal cells, the subcellular compartments in which the wild-type and mutant forms are processed and localized, and the functional alterations which occur in spiny cells expressing the mutant transcript. Information on these biological properties of huntington is vital to understand the pathogenesis of HD and to develop a therapy for treating the disease. Our overall hypothesis is that in the neostriatum the IT15 mutation modifiers the cellular processing and localization of huntingtin preferentially in spiny cells, thus leading to abnormal protein interactions that are functionally harmful to these neurons. Immunohistochemical and in vitro transfection experiments are planned to examine the localization and functional properties of huntingtin. With well-characterized antisera, huntingtin localization will be examined in human control and HD presymptomatic, homozygote, and grade 1-4 HD brains (Aim 1), and in the developing and adult rodent striatum (Aim 2). Transfection of epitope-tagged transcripts into hybrid striatal cells will be used to determine the effects of expression of the mutant form of huntingtin on the transport and processing of the protein (Aim 3a), and on the survival, neurite outgrowth, and response to excitatory amino acids (Aim 3b). These studies should help to define those characteristics of huntingtin neurobiology and its mutation that underlie the marked loss of striatal neurons in HD.